Draft tube



L. F. MOQDY DRAFT TUBE May 17, 1932.

Filed May 8, 1925 8 Sheets-Sheet 1 May 17, 1932. F. MOODY DRAFT TUBE! Filed May 8, 1925 8 Sheets-Sheet 2 I! I I l 1 z r a 1' i 1 l 1 I 1 i i Q I N 1 Y a I 3 N l 3 5' K &

Jvwentoz Ftp ASi mlzd Q May 17, 1932.. L. F. MOODY 1,859,215

DRAFT TUBE Filed May 8 1925 8 Sheets-Sheet 5 Durance on a;

awuemto'c L. F. MOODY May 17, 1932.

DRAFT TUBE Filed May 8 1925 8 Sheets-Sheet 4 May 17, 1932. L. F. MOODY 1,859,215

DRAFT TUBE Filed May 8, 1925 8 SheetsSheet 5 May 17, 1932.

L. F. MOOD DRAFT TUBE Filed May 8 1925 8 Sheets-Sheet 7 .P 1 7 F g A QHIOWWS L. F. MOODY DRAFT TUBE Filed May 8 1925 8 Sheets-Sheet 8 lll VENT

charged in another direction at an angle to Patented May 17, 1932 UNITED STTES LEWIS FERRY MOODY, F PHILADELPHIA, PENNSYLVANIA DRAFT TUBE Application filed May 8, 1925. Serial No. 28,868.

This invention relates to draft tubes for hydraulic turbines and-the like and particularly to draft tubes wherein the flow received in one direction is turned and disthe first.

The object of the invention is to provide a draft tube which will smoothly and efficiently decelerate the flow within a. relatively small space. The smoothness of the deceleration not only helps in increasing officiency but also avoids any tendency toward vibration sometimes set up by a flow at high velocity or under reduced pressure. The close spacing of a series of turbine units in a powerhouse also frequently limits the dimensions of the draft tube, particularly in transverse width, so that there is no room for extensive lateral spreading of the tube without interferen'ce with adjacent units or requiring a prohibitively wide spacing of the units.

I have found that by bending the discharge stream as a whole and at the same time changing its cross sectional form and properly controlling the rate of velocity deceleration throughout the stream, the regular and efficiently smooth turning of the entire flow may be attained within very narrow space restrictions, in a conduit of simple construction.

In the accompanying drawings illustrating the invention Fig. 1 is a sectional view through a draft tube illustrating the invention.

Fig. 2 is a plan view of the same with successive sectional shapes indicated at corresponding points of the tube, each cross section indicated at I, II, III, IV, V, VI, VII, VIII and IX of Fig. 1 being indicated in outline turned into the horizontal plane.

Fig. 3 is a typical graph showing the velocity of the flow at successive points in the tube.

Fig. 4 is a diagrammatic representation of a comparative straight conical tube having the same velocity values as those in the bent tube of Fig. 1, shown in half section.

Figs. 5 and 6 are views similar to Figs. 1 and 2 but illustrating another form of tube. Fig. 7 is a vertical section of a modified form of tube taken on the line 7-7 ofFig. 8. Fig. 8 is a sectional viewtaken on the line 8-8ofFig. 7. 1

Figs. 9'and 10 are partial diagrammatic views showing the changes inthe form of the-tube of Figs. 7 and 8 at successive sections indicated on the figures. Fig. 11 is a diagrammatic development of the line A, G, F, E, D indicated on Fig. 8.

Fig. 12 is a'sectional view of a further odified form of-tube taken on the line 12-12 of Fig. 13. a r i Fig. 13 is a sectional view taken on the line 13-13 of Fig. 12, and 1 Figs. 14 and 15 are sectional views on the lines 14-14 and 15-15 respectively of Fig-13. e

In the embodiment of the invention shown in Figs. land 2 the flow leaving the turbine runner enters the draft tubeat 20, is turned in the bending portion 21 0f the tube and passes out to tailwater through the discharge portion 22. i i

From the entrance section, designated I at the center line, to the section II the tube is substantially straight and circular in section. Beyond section II the curvaturedefinitely begins and continues with a center line designated II, III, IV, V, VI and VII.

Throughout the turn the sectional form pro gressively changes, as shown in Fig. 2, from substantially circular at section II to a transversely elongated section at VII. The side walls 23 and 24 spread or diverge (Fig. 2) while the inner surface 25 and outer surface 26 of the bend (Fig. 1) at first remain nearly the same distance apart or slightly diverge to III and then gradually converge so as to approach nearer and nearer together. This permits the inner surface to curve gradually with a large radius of curvature and likewise the outer surface 26 curves along a sweeping gradual bend, and the stream between surfaces follows its natural tendency to spread laterally while its inner andouter surfaces converge. Each element of the flow thus smoothly bends and changes in shape in a natural manner and without tendency to set up cross currents or eddies.

This change in direction and form of the stream is accompanied" by a reduction in the velocity so as to simultaneously convert velocity head into pressure head. At first this deceleration of the velocity may be relatively large and approach closely to the permissible efiicient decelerationinIa. straightv draft tube. As the tubezcurves however the rate of deceleration is reduced throughout the major portion of the bend. As shown in the diagram Fig. 3, for -instance,- plbtti'ng velocityagainst distance along the center line, the

velocity is quite rapidly reduced until near J curve, the less will be the likelihood: of. in-

regularities inFthe flowor. any separation of the flow from-the tubewalls. Themoreirapid the curvature of the tube Walls the? less will bethe tendency; of the/flow to follow them. Consequently the portions of aibend that are relativelyabrixptand:of small z'radius of curva'ture"willzhavetherleast deceleration andrthe more rapidly the radius of curvaturedecreases. thermore rapidly will the rate of deceleration decrease.

In the process-of" decelhrating the velocity of flow of waterinia passage enl'angingin the direction 0t flow; the: angle of" divergence of the side walls from. the initial direction of flows is muchimone*efi'ectiveonrthe elements of tliefiowing stream which are adja'cent to':the side Walls, than! it is upon: the central porthe: side walls and: therefore do: not; feel" the f influence oftheirangle to the same extent.

Ilhe result isthat in a straight, enlarging cond'uit the'centrah elements 0t the-flow are decelerated much. more slowly and less? effecti vely than the GlBHIGIItSIyIII'COIltEIC-t Withthe Wall and: therresulti of the! enlargement is therefore to set upanunequal: distribution of velocity across the conduit, the velocityv at the center remaininghigher' than at the side walls If the angle of divergence of the side Walls is too great,'. this unequal distribution of flow may; becarried so far. as to result' in eddies or-backward flow around. the walls with: as consequent loss of energy and. ineflicicnt deceleration Inna conduit containing; abend or'elbow, aisi-milarq eflteet 'is caused even "Vhen the area of: the c'ond uit' remainsconstant in passing around: the elbow.- The efiectof' the bending of: the streami Is torset up centrifugal. forces which build up "an: increase of pressure around the outside of the bend and cause a reduction of pressure at the inside of the bend with a consequent tendency toward an increase of velocity at the inside of the bend and a decrease at the outside, so that the effeet of a bend in a channel is to cause a grea er andgreater. inequality of velocity distribution asthe flow passes around the liend. If the curvature of the conduit is too rapid; the flow will end to leave the inner surface of the bend leaving eddies on'bacltward flow in this region, or an efi'ect similar to too rapid enlargement of a straight conduit. It is thereforeseenthat. the eii'ect of gradual enlargement in a ccnd-uit'and the effect of curvature oi? a=.-.conduit without. enlargement are similar in: creatingnoneuniform distribution.- of. ve: locity transverse. to the how. g I

Since theeffects of? enlargement and of curvatureof'the: centerlineof. the conduit are thus in general similar in creating norr-uniform distr-ibutionaofvelocityaacnoss;assection, it 3 follows that. when: we desire both to; deeeL e-rate velocity. and to turnritaround a! bend at the sametime it. is; necessary tolimit the rate of enlargement in; proper relation to-the rate of curvature-oi the conduit so that the two: clients when superimposed. will not result in extreme variation. of velocity across the-conduit or the? Great? on of backward flow or-eddiesa. V v A-sthe-flow priogresses'around a-bendi er 'elr bow of. the-conduit,therefore, itis necessary progressivelyto decrease-the rate -ot deceleratlOIIrlII' passing from the-entranceto thedischargersection of the bend and: at the; point where'the greatestdistortiomof: flow would be caused by the; cunvatuneof the conduit, for instance, adjacent the discharge end: of the elbow, itv is; necessary to 7 reducethe rate of deceleration-tea very low value approaching. a.- condition of constant velocity. Atter the How has been deflected into anew'direc- .tiongthe rate-of deceleration: canthen be gradually increased so that at points more distant front the. elbow the: rate 0t. deceleration can approach more andinorecloselyto that permissible in. a straight conduit In Fig. 4 thes-uccessive areasot the tube of Fig. 1: are represented by the radii? of equivalent circular sect-ions laid oif along a straight center lineso-as to show the form of the Wal'lof an equivalent straight-tube 0t circular: cross section. This indicates a relatively large angle offlare Uz at. entrance l substantially straight portions of the tube have a relatively large angle of flare -for instance corresponding to 5 or even 7 in an equivalent straight tube such as indicated in Fig. 4. The bend of the tubehasas much velocity reduction as can be effected during the process of deflecting the flow through an angle, consistently with the maintenance of smooth conditions of flow. I have found that while these conditions do not permit a high rate of deceleration at the bend the average permissible deceleration, and consequently the total deceleration, in the bend may be considerable and consistent with the maintenance of highly smooth and efiicient flow conditions. In Fig. I, for instance, the flare of the straight tube walls corresponding to the bend of the tube of Fig. 1 decreases to be tween 1 and 3. and then slightly increases as the end of the elbow is approached and the curvature of the passages decreases. This flare around the curve gives an increase in area between the entrance section II and the discharge section VII so that the discharge area VII is equal to 1.7 5 times the entrance area II and in general the discharge area of the bent portion of the tube will be from one and one-half to two times the entrance area and the velocity at the discharge VII will be about two-thirds to one-half of the velocity at the entrance section II.

Simultaneously with this increase in area the cross sectional shape of the tube changes to widen out laterally and decrease the distance between the inner wall and outer wall so that the distance 6 between these walls at the discharge VII is less than the dimension (1 of the substantially circular entrance section II and in general this dimension 5 will be not over two-thirds of dimension a, nor less than one fourth of a.

The shape of thebend in plan view (Fig. 2) will vary depending for instance on the available space for the widening of the bend at the turn. Where permissible this lateral spreading may be carried out to comparatively large widths as shown for instance in Figs. 8 and 13, but the conditions of installa tion will often make it advisable or necessary to retain the lateral width of the bend within dimensions permitting only a relatively gradual lateral spread. This will be the case when a portion of the draft tube structure is already in place in the plant or where it desired to modify an existing draft tube so that its form and area transition may be made to conform to the principles here explained.

In the bend shown in Figs. 1 and 2 the tube expands laterally with a moderate increase in width frompoint to point as indicated by the side surface contour lines 28 and this lateral expansion is such as to give these contour lines 28 an outward convexity away from the axis at the entrance sections I, II, III followed by a fiexure leading to a convexity toward the the final outlet passage 22. The outlet passage 22, asshown in Fig. 2, expands laterally at a lower rate than the lateral expansion of the bend, or as shown for instance in Figs. 8 and 13 the outlet passage will not expand laterally at all.

In case it is permissible to widen the draft tube passage, the same successive area and velocity relations may be maintained around the bend by bringing the inner and outer surfaces 25, 26 closer together.' This will reduce the distance from the inner to the outersurface and may change the side contour lines to be generally convex away from the center as indicated in the side contour -linesof Figs. 8 and 13. 1

The inner surface 25 of the bend isshown as are shaped in section II and flat with transverse straight line elements of increas ing length in sections III to VII where it merges into the flat roof 29 of the outlet passage 22. As indicated in Figs. 7 to"15 this inner surface of the bend may be transversely curved instead of flat, this transverse forma' tion being designed to permit sufficient vertical contraction to be provided for the portion of the stream which passes directly downstream in the center of the'discharge portion, while at the same time providing enough discharge area for the laterally deflected portions of the stream by increasing 1 the height of the passage as the sides are approached. The resulting formation is characterized by the inner surface of the elbow changing from a shape concave inwardly at the beginning of the bend to a form convex inwardly at the end of the bend, when frliewed in planes normal to the direction of By this combination of changing section shape and similar bending of the flow lines across the tubeand the progressive decrease in the rate of deceleration, the flow received at the entrance is turnedand has its velocity head converted into pressure head within laterally narrow space restrictions and with maintenance of efficient and smooth conditionsof flow. The result is that the draft tube of this invention co-operates with the turbine to increasethe net head and the overall efficiency, while at the same time avoiding any tendency to set up vibrations in the water column passing through the tube. y

Figs. 5 and 6 are views corresponding to Figs. 1 and 2 and illustrating a modified tube with decreased lateral spreading of the tube around the bend. In these Figs. 5 and 6 the reference numerals of Figs. 1 and 2 indicate corresponding parts by corresponding numerals primed. The inner and outer walls 25', 26 of the bend of this modified tube do not approach each other as rapidly as the V 4 1 nesogaris walls 25,. 26; ofthe' draft. tube of Figs 1,. and thesectionsIII; IV, V, VI and VIIof: the modified tube are. reduced in lateral width, the. successive areas and velocities being clbsely analogous in. the two tubes.-

In the modification illustrated in Figs. 7 to '11: the draft tube comprises a straight portion. 39*receiving the flow and deceleratingrit and passing it to the curved portion 41 where the;flow'issimultaneously turned, spread and decelerated. From this curved portion the flow is passed'tothe' outlet portion 42. The berrdr tiofithis-tube has a; wide lateral spread of the side surfaces 48 (Fig. 8) and the inner snrface 45and-the outer surface 46 approach toward each other at a comparatively rapid ratesothat the 'central'dimension b at the-end of the bend i's-lessthan half the diameter mat the entrance. The inner surface 15 forms part of a. bell shaped flare formed by the spreadingof the walls of the bend andcurving'smoothly downwardlyand outwardly to a rounded ridge or inverted crest formation -50-of' generally ovalshapeas indicated by the dot and dash. lines 50.- of Fig. 8. Beyond the end- 50 of this fl'are the curvature of the surface'continues to form an upwardly and outward-1y incl-inedroof surface 52.

I The floor 51 of the draft tube is flat and the lowermost portion 50 of the flare approaches closely tothe floor at the center as indicated in the develop-ed'view of. Fig. 11 where the'contour surface 50 with relation to the floor 51 is shown laid out in a plane. The. vertical dimensions 79' are indicated at the points A, G-, F, E and D corresponding to these points as indicated in Fig. 8. This flare formation is also shown in the Vertical sectionsof Fig. 9 showingsuccessive-sectional contours-oneach side along. the radial planes QC, OD D- and 0E 11 and the combining radial and transverse planes OFFand OG G. These sections show the rapid flaring of the bend'at the successive sections and. the continuati'onof: the'flare to form theroof' surface 52 continuing the lateral expansion and merging wit-h the roof surface 53 of. theontlet portion42. As shown in Figsv7, 8 and 9 the roofsurface 52 on each side expands and passes downwardly around the crest line 50 andiwith transverse elements (Fig 9) having an upward and outward inclination beyond the line 50.- The flow lines are thus: spread carefully controlled as explained in connec tion with Figs. 1- t0'4. so as to'progressively lower: the rate: of. deceleration: during. the turning. of the flow. The deceleration and turning of'theflow isthus smoothly and efii; ciently accomplished'within moderate lateral dimensions and: very'short vertical distance and roof 53 shaped as shown in Figs.- 7 and 8" and the successive sections HH, II and KK of- Fig. 10. The downward bulge of the surface 50 givesa downward or inverted bulge: orridgeto the-r0055? at the section HH and the tendency of success-ive sections of. the passages is to flatten out this bulge toward the outlet end of: the tube. A piermem'ber'55 is provided in the outlet section; as shown and: extended any desired distance toward: or. eveninto the bend 4:1 of the tube.

In Figs. 12to 15' a further modification isshown with as still further widening. of: the

bend ofthe tub-e,.the reference numeralsof Figs? to 11 beingprimed. and. conne'ctedito corresponding elements of Figs. 12 to 153. In. this modification of Figs. 12 to 15 the rounded ridge of the inverted crestf50 is substantially horizontal throughout and the A roof surface 53. continuing; from this; ridge is bulged downward atvthe center, as indicated' in. F ig: 15, and is: inclined upwardly and outwardly away fromtthe substantially horizontal floor surface; 51. as indicated in Fig; 12;. The flare 'surfa'cese xtend. outward so? widely that a. transverse section such as 14: at the center of. the tube shows these surfaces merging with and tangent. to the roof surface. 52 closely adjacent the side surfaces 48. Fromathis section 14: toward tailwater' the. roof surface 52." on: each side will incline upward at a moderatean'gle as in the draft tube of Figs. 7 to 11, and as indicated in Fig. 15 forms a: continuation. of the. roof surface 53.

The widely spread form of tube shown in Figs. 7 "to 15 is advantageousv in that it permits a large portion" of the flow to have its meridian velocity components diverted into radialor'approximately radial directions so that these components are. turned outwardly away from the axis on all sides; except on the upstream side of the draft tube away from the tailrace. an annular form in which: the water-is per= mittedto continue-its rotation about the turbine axis while it passes outwardly to successively greater distances from the axis. In this way the whirl components of the stream leaving the runner can be efliciently deeelerated. duerto the principle of constancy of momentum, according. towhich the velocity of whirl will. decrease in inverse proportion to the distance from the axis.

The passage thus approaches This widely flaring draft tube is, there fore, particularly applicable to turbines of gradually upward and outward until it intersects the side walls, and thus provides a collector passage around e'achside of the inverted crest of the roof. The roof of the collector atv the same time slopes gradually upward towards the tailrace, thus decelerat ing the laterally deflected portionsof the stream. 7 V The draft tube of this invention is also particularly advantageous in not carrying the excavation upstream any farther than necessary, so that the power house foundation is not unduly cut into. In many plants, and

inside surface 25, and 45, of the bend to give a gradual turning of the stream where the velocity is high and the pressure low as is the case along this surface. The radius of the outer surface of the bend must also, of course, not be so long as to impair the proper relative spacing between the inner and outer surfaces and the desired sequence of areas around the bend as pointed out above. The ratio of the radius of curvature 1" of the inner surface of the bend in'the plane of the bend to. the diameter a at the beginning of the bend, that is,

. will be approximately as follows:

In a tube having a wide lateral extension in which the low point of the bell extends around through about 180 of circumference, as in Figs. 7 to 15,

will have a minimum value of approximately .60, and this radius of curvature 1" should not be substantially less than .60 of the initial diameter in any of these tubes. In tubes hav- 7 and inefiicient deceleration.

ing a smaller lateral width such as'shown in Figs. 1 to 6, I

may reach values as high as 1.5, corresspondingto a long and deep tube so that in most cases the radius 1 will not greatly ex ceed the diameter a.

' The roof of the side collector passages of Figs. 12 to 15, beginning just upstream from 52 in Fig. 14, slopes upwardly toward the tailiace in straight lines making aniangle of about ten degrees with the 'lioor, an angle which is consistent with etlicient deceleration of the flow When the downstream length of the discharge passage is greatly restricted this angle can be slightly increased but should not exceed about 15 to avoid loss of head The central portion of the roof as shown at 53 in Fig. 12 slopes upward at a similar angle; but since the side portions begin farther upstream the roof at either side of the discharge passage will have reached at the section shown "by ldlg. 15 a higher elevation than at the center; so that in Fig. 15 and other succeeding sections between Fig. 15 and final discharge the horizontal discharge passage will have a.

downwardly convex roof; while in Fig. 12 and sections parallel to Fig. 12 theroof will have straight line contours inclined to the floor at a small angle. a

In addition to the pier member 55"in the draft tube of Figs. 12 and 13 stay vanes may be added to support the roof from the floor of the draft tube, these stay. vanes hav: ing flange end portions 61 embedded "in the concrete of the substructure. The stay vanes are formed and positioned to olfer a minimum resistance to the flowand are preferably placed in substantially semi-circular its formation adj acent the downwardly directed V crest 50 or 50.

While this invention has been illustrated and described in connection with a vertical shaft turbine and with the draft tube turning the flow toward the horizontal direction, it is obvious that the invention may be used in connection with turbines having inclined or horizontal shafts and is not confined to the specific embodiments shown and de-.

scribed.

In a horizontal shaft single or double runner turbine the entrance portion of the tube will in" general be horizontal and the discharge portion directed vertically downward.

This form of tube is advantageous in its increasing in cross sectional areaat a decreas ing rate in the direction of flow,

2. A draft tube comprising a bent portion increasing in cross sectional area at a con tinuously decreasing rate in the direction of Q I 31A drafttube'comprising a bent portion increasing in cross sectional area at a decreasing rate inthe direction of flowj and with decreasing distance between the inner and outer surfaces of 'the bend. x 1 k i. A draft tubecoi'nprising a bent plortion increasing inci'oss sectional area at'a decreasing rate in the direction of flow and with constantly decreasing distance between the inner and outer surfaces of the bend; I

15. A draft tubecolnprising a bent portion increasing in cro'sss ectionalarea at a decreasingrate in the direction offiow and with the radius 'of curvature of the inner surface of thebe'ndnot'less than six-tenths of the dianr etero f'the tube at thebeginning of the bend. A draft tube comprising abent portion increasing incross sectional area at a decreasi'n-gr'ate inthedir'ection of'flow and with the radius of'curi a'ture' of the inner surface of thebend notfle'ss'than six-tenths 'of'the diameter ofth'e'tube at the beginning of thebend and the radius of cur'Vature-ofjthe outer surface oftheb'end being greater than radius of curvatureof the inner surface of the bend.

A draft tube-comprising a-bent portion increasing in cross sectional area at a demeasured along the respectiye centerlines the creasing rate in the direction offio'w and with decreasing distance between the inner and" outer surfa'cesof the bend, the'c'ross sectiona'l shape "of the tube changing fro-1n su'b st'antia'l'ly circular" at 'theentrance to'the bend to "a laterally elongated shape at the outletof the bend.

"'8'. Adraft 'tube'comprising' a "bent portion increasing in cross-sec'tiona'l area at a decreasing "rate in the 'directionof flow, and a discharge portion increasing in cross-sectiona1 area at an increasedrate.

A draft tube-comprising a bent portion increasing in cross-sectional areain'the direction of flow at a decreasingrate, said rate reac'hinga minimum Value nearthe discharge end of said begnt portion.

I :10. draft tube comprising a bent :portion followed fby a discharge portion, the cross sectional area of said tube increasing in said bent portion at a progressively decreasing rate in the direction of flow until the rate reaches a small minimum value near the end ofsa'id'ben't portion, the area then increasing at an increased rate in the discharge portion.

'11. A draft 'tu'be'comprising'a bent portion foll'owed by a"relati vely straight dischargeportion, said tube having cross-sectional;-- areas such "that -*in an equivalent straight conical tube haying circular areas equal to those of said tube at equal distances tion at first increasing in cross sectional area at a decreasing rate in the direction of fiow andthen increasing in cross sectional area-v at an increasing rate before theendof the bend is reached.

18. A drafttubecomprisingabentportion at first increasing in crosssectional areaat a decreasingr-atein'thedirection of flow \and with decreasing distance between the inner and outer surfaces of the bend 'and then increasing in cross sectional area at-an increasingrate before the end of the'bend is reached.

14. A draft tube comprising a bent portionv at first increasingin cross sectional area at a decreasing rate in the direction of flowand with constantly decreasing distance between the inner andoutersurfaces of-therbend, and then increasing .in cross sectional area at an increasing rate before theendof the bend is reached. v M

15. A draft tube comprising albentiporti'on increasingincross sectionalareaat adecreasing rate in the direction of flow so that the discharge area at theoutletbf the bend is from to twice the area, at the entrance to the hen 16. A draft tube comprising a bentportion increasing in cross -sectional area at a continuously decreasing rate in the direction of flow so that the dischargeareaat the outlet of the bend is from.% to twicethe area of the entrance to the bend.

'17. A draft tube comprising a bent portion increasingin cross sectional area at a decreasing rate in the direction of flow and with decreasing distance between the inner and outer surfaces of the bend so zthat the discharge area at the outlet of thebend .is from to twice the area at theentrance to the bend. p

18. In a draft tube a bent portion of substantially circular cross sectionat entrance and laterally elongated shape at discharge,

the inner and outer surfaces of the bend 1 gradually curved inner and outer surfaces continuously approaching each other in the direction of flow and with laterally spreading sides, and a discharge portion having the continuations of said inner and outer surfaces diverging from each other at an angle of not over 15 while the side surfaces of said discharge portion are substantially parallel.

21. In a hydraulic turbine installation a draft tube having a vertical entrance and horizontal discharge, the entrance portion comprising a surface flaring outwardly from its axis, the downstream portion of said surface curving downwardly and outwardly in meridian sections to a point of minimum elevation and then rising again so as to form an inverted crest, said crest extending, with uniform minimum elevation, circumferentially around the axis of said entrance portion so as to subtend an angle of at least 180 at said axis, and the upstream portion of said surface smoothly curving from the entrance direction to the horizontal discharge direction with a radius of curvature not less than the radius of curvature of the downstream portion. 7 c

22. A draft tube comprising a vertical entrance portion and a horizontal discharge portion; said vertical portion having on its downstream side an outwardly flaring surface of revolution extending around the axis of the vertical portion through an angle of at least 180; and said horizontal portion having a. flat horizontal floor and having a roof surface composed of straight line elements sloping diagonally upward toward the discharge at a small angle to said floor, and the upstream portion of said surface smoothly curving from the entrance direction to the horizontal discharge direction with a radius of curvature not less than the radius of curvature of the downstream portion.

23. A draft tube comprising a vertical entrance portion and a horizontal discharge portion, said vertical portion having on its downstream side an outwardly flaring surface of revolution extending around the axis of the vertical portion through an angle of at 'i least 180, and said horizontal portion havupstream portion of said surface smoothly curving from the entrance-direction, to the horizontal discharge direction with a radius of curvature not less than the radius of ourvature of the downstream portion. r 24. A draft tube comprising a vertical en'- trance portion andahorizontal discharge portion, said vertical portion having on its downstream side an outwardly flaring surface of revolution extending around the axis of the vertical port-ion through an angle of at least 180, and said horizontalportion havr ing a flat horizontal floor and having a roof surface composed of straight line elements sloping diagonally upward toward the dislc iharge at an angle of notover 15 to said oor.

25. In a hydraulic turbineinstallation a ual curvature (or large radius curve) into.

the floor of the discharge passage and with the inner part of the surface curving downwardly and outwardly to a point of minimum elevation and then sloping upwardly and outwardly to form an inverted crest, said crest extending, with uniform minimum elevation, circumferentially around the axis of the entrance portion so as to subtend an angle of at least 180 at said axis. 7

26. In a hydraulic turbine installation a draft tube having directions of entrance and discharge approximately at right angles and comprising a bent or elbow portion followed by a substantially straight discharge section,

the portion of its surface toward the inside I from being inwardly concave to inwarc ly' convex when considered in planes normal to the direction of flow and said inwardly convex surface continuing downstream completely to the discharge endof the discharge section to form a relatively long and gradually enlarging dis-charge passage. i 27. A draft tube comprising vertical and horizontal portions of generally elbow formation, said horizontal portion having its roof surface curved when out by a substantially vertical transverse plane, to provide an inverted ridge, said ridge extending completelyto thedischarge end of the draft tube so that the final discharge section of the tube has a roof surface of varying height.

28. A draft tube comprising vertical and horizontal portions of generally elbow formation, said horizontal portion having itsv root surface curved whencut by a-sublstantially vertical transverse plane, to. provide. an inverted ridge substantially at the central.

plane of the horizontal p0rtion,-.the bottom 8 igssegers surface of "said ridge sloping gradually up ward :in substantially straight lines to the discharge end of said horizontal portion so as'to subte'nd an angle of not over 15 to the 'fioor of said horizontal portion.

29; A draft tube having horizontal and vertical parts, said horizontal part being of greater depth near one of its sides than at other points disposed progressively inwardly from apoint'near said side, and said varying V vertical parts,- said horizontal part being of greater'depth near one of its sides than at other points disposed progressively inwardly from a point near said side, and said varying depths being bounded by a relatively smooth and continuous surface in a direction transverse to the flow, and said surface being subs'tantially straight inthe direction of flow and the'depth of the discharge portion of the horizontal part gradually. increasing in the direction of flow so that the upper and lower surfaces diverge from each other at an angle nowhere exceeding 15 31. .A draft tube for a hydraulic turbine comprising an entrance portion directed in the direction of axial discharge from said turbine, and a discharge passage directed substantially perpendicularly to said entrance portion, the walls of said discharge being connected to the walls of said entrance portion by surfaces of smooth and continuous curvature to form a generally elbow shaped passage, said discharge passage having a dimension measured. ina direction parallel to direction of flow, its length from the point the turbine axis which is less at a pointnear the central plane of the elbow than at a point laterally displaced from said plane and near a side of said passage, and said discharge passage being of substantial length in the and the'discharge end of "the passage being at least equal to the distance of said'point from the turbine axis.

32A draft tube for hydraulic turbines, comprising entrance and discharge passages, theroof of said discharge passage being substantially gradually inclined downwardly from opposite sides toward the center so that said discharge passage in transverse section substantially gradually and smoothly decreases: in depth toward the center, the length of; said discharge passage from its point of minimum depth to its discharge endbeing substantially greater tha'niits greatest depth.

33. A draft tube comprising a bent portion followed by a relatively straight discharge portion, said tube havingcross-sectional areas such that in an equivalent straight conical tube having circular areas equal to those of said tube at equal distances measured along the respective centerlines the angle-between the wall of the equivalent straight tube and its centerline will progressively decrease from an initial value of not less than from. 5 to 7 to a minimumv value at a point nearly corresponding to the end of the bent portion, said decrease in angle from initial to minimum value amountingto at least 5,and said angle then increasing in the discharge portion to at least 7.

34. 'In a hydraulic turbine installation a draft tube comprising an entrance-passage having a vertical axis and a'horizontal discharge passage connected by a bend or elbow portion, said bend being formed with the outer part of its surface curving With gradual curvature (or large radius curve) into the floor of the discharge passage and with the inner part of the surface curving downwardly and outwardly to a point of minimum elevation and then sloping upwardly and outwardly to form an inverted crest, the depth of the passage at saidpoint of minimum elevation being lessthan one-half the diameter of the entrance passage.

35. In a hydraulic turbine installation a draft tube comprising an entrance passage having a vertical axis and a horizontal discharge passage connected by a bend or elbow portion, said bend being formed with the outer part of its surface curving with gradual curvature (or large radius curve),into the floor of the. discharge passage and with the inner part of the surface curving downwardly and outwardly to a point of minimum elevation and then sloping upwardly and outwardly toform an inverted crest,the maximum depth of said dischargelpassage at its smallest section being materially less than one-half the width of said smallest section.

86. A draft tube comprising vertical and horizontal portions of generally elbow formation, said horizontal portion having its roof surface curved when out by a substantially vertical transverse plane, to provide an inverted ridge, said. inverted ridge when out by a vertical longitudinal plane having a straight contour for a substantial distance in the direction of flow in order toprovide a relative long gradually enlarging discharge passage. Y

' LEWVIS FERRY MOODY.

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